The invention relates to measurements relating to the composition of oil well effluents, constituted by multiphase fluids typically comprising three phases: two liquid phases, namely crude oil and water, plus a hydrocarbon gas phase, and more particularly to measurements of gamma ray attenuation by the fluid. The invention also relates to the association of such measurements with flow rate measurements in order to determine the flow rates of the various phases.
In the oil industry, the traditional practice is to separate the effluent into its component phases and to perform measurements on the phases separated in this way. However that technique requires separators to be installed on site, which separators are bulky and expensive items of equipment, and when testing wells, it also requires additional pipes to be put into place.
Numerous proposals have been put forward for developing techniques that would make it possible to avoid using such separators. A description of these developments is to be found in SPE publication 28515 (SPE Annual Technical Conference, New Orleans, Sep. 25-28, 1994) by J. Williams, xe2x80x9cStatus of multiphase flow measurement researchxe2x80x9d.
Amongst such proposals, U.S. Pat. No. 4,788,852 describes apparatus including a device for measuring gamma ray attenuation, that device being associated with a Venturi total flow rate sensor and being situated at the constriction of the Venturi. Apparatus of that type is also described in patent application WO 94/25859 and in SPE publication 36593 dated Oct. 6, 1996, xe2x80x9cMultiphase flow measurement using multiple energy gamma ray absorption (MEGRA) composition measurementxe2x80x9d by A. M. Scheers and W. F. J. Slijkerman.
The invention seeks to provide such gamma ray attenuation measurements in an advantageous manner, that is particularly well adapted for being associated with flow rate measurements using the Venturi effect.
In one aspect, the invention provides a method of characterizing an oil borehole effluent, formed by a multiphase fluid mixture which typically comprises water, oil, and gas, comprising the steps of emitting gamma rays at a first energy level of about 100 keV and at a second energy level of about 40 keV, and measuring the attenuation of the gamma rays at these two energy levels after transmission through the effluent.